This invention relates to a stereophonic transmission process involving the use of a dummy head and headphones and to means for carrying out the process.
These types of head-related transmission systems are provided with a dummy head simulating a human head at the start of the transmission chain and headphones at the end of the transmission chain. The following transmission system has entered practical usage: at the place of sound pick-up a dummy head is located which reproduces as well as possible the total acoustical conditions existing at a natural head (for example a dummy head as described in German Pat. No. 19 27 401), that is to say the signals from the microphones of this dummy head should correspond as accurately as possible to the signals at the ears of a natural head. These microphone signals are recorded on a sound recording medium (for example a record or tape) and/or transmitted on a transmission path (for example a stereophonic broadcasting channel). Sound reproduction takes place by means of commercially available headphones.
Head-related transmission systems of this type have a number of grave defects which have led to the situation that this transmission method, which, in itself, is very efficient, has hitherto found only little acceptance:
(a) In the form described, head-related stereophony is generally thought to be not compatible with conventional, space-related stereophony, that is to say both the reproduction by loudspeaker of the signals from dummy heads and the reproduction by headphones of conventional space-related stereo signals lead to auditory events which show transmission errors both with respect to space and to sound.
(b) Hitherto, no dummy heads have been designed which have transmission properties which correspond accurately enough to the basic design principles of these heads. For this reason, there is also no standardization for the communications-engineering-related transmission characteristics of dummy heads such as is in the natural order of things, for example, for microphones in space-related recording engineering. One consequence of this is that the transmission functions of the dummy head and the headphones are not matched to one another.
(c) The signal/noise ratio of the usual signals from dummy heads is too low in certain ranges of frequencies, so that the microphone hiss becomes audible although reproduction is correct in other respects.
(d) Head-related stereophony in the form described does not offer any possibility of individual matching, that is to say the differences, which are important for head-related transmission, in the directional pattern of the dummy head and of the individual listener are not taken into account.
Defects (a) to (d) are reflected in transmission errors with respect to localization, tone and freedom from interference of auditory events. As remedy, the following further developments have become known which, as partial solutions, relate in each case to one of the defects listed:
with respect to (a), loudspeaker reproduction of head-related stereo signals: German Offenlegungsschrift No. 25 39 390; headphone reproduction of space-related stereo signals: U.S. Pat. No. 3,088,997, German Offenlegungsschrift No. 20 07 623, German Offenlegungsschrift No. 22 44 162, Acustica Vol. 29 (1973) pp. 273-277, German Offenlegungsschrift No. 25 57 516, Radio Fernsehen Elektronik Vol. 28 (1979), Number 4, pp. 222-225;
with respect to (b), transfer functions of the dummy head: Rundfunktechnische Mitteilungen Vol. 22 (1978), pp. 22-27, Fortschritte der Akustik (advances in acoustics), VDE-Verlag Berlin (1978), p. 645;
with respect to (c), signal/noise ratio: Acustica Vol. 41 (1978), pp. 183-193;
with respect to (d), individual matching: U.S. Pat. No. 4,143,244.
Even if these further developments are taken into consideration, however, problems (a) to (d) are not solved [(b), (c)] or are solved inadequately [(a), (d)]. In addition, the known technical solutions for (a) and (d) are too elaborate in comparison to the improvement achieved by them. All the further developments suffer from the fact that they attempt to solve their respective partial problem in isolation and do not take into account the effects on the total head-related transmission system, including compatibility with conventional space-related stereophony. The result of this isolated type of approach has been that either partial problems were not appropriately set within the context of the total transmission task [(a), (b)] or that partial solutions were arrived at the expense of other partial problems [(b), (c)].